A direct fuel cell (DFC) is known as one of various types of fuel cells. For example, a direct methanol fuel cell (DMFC) using methanol as a fuel compound has a high energy density and is expected as a next-generation small power source. A chemical reaction in the DMFC is as follows: anode reaction: CH3OH+H2O→6H+6e−+CO2; cathode reaction: 1.5O2+6H+6e−→3H2O; and overall reaction: CH3OH+1.5O2→2H2O+CO2.
However, a phenomenon called methanol crossover (MCO), in which methanol supplied to an anode cannot react completely in the anode reaction, and unreacted methanol passes through a proton-conducting electrolyte membrane to move to a cathode, is becoming a problem.
In general, a platinum catalyst is used as a cathode catalyst for the DMFC. The platinum catalyst promotes not only an oxygen reduction reaction but also a methanol oxidation reaction. Therefore, methanol that has undergone crossover causes a methanol oxidation reaction in a cathode as well, which remarkably degrades power generation performance of the DMFC.
In view of the foregoing, conventionally, platinum and other noble metals such as palladium (Pd) and ruthenium (Ru) which promote only the oxygen reduction reaction have been used in combination as a cathode catalyst (see, for example, Patent Literatures 1 and 2).